Field of the Invention
The present invention relates to a wiring substrate, and more particularly to a wiring substrate onto which solder having a low melting point is bonded and a method for manufacturing the same.
Description of the Related Art
In a wiring substrate, such as a semiconductor chip mounting substrate provided with an electrode consisting of Cu or a Cu alloy and a printed wiring board, a multilayer wiring substrate using a build-up method is widely applied, in order to accommodate higher frequency, higher density wiring, and higher performance of a wiring substrate. Also, in order to realize a smaller, thinner, and lighter product, each electronics manufacturer attempts to deal with high density mounting, and as a result a package has had greater number of pins and narrower pitch. Particularly, the mounting on a printed wiring board has been developed from a conventional Quad Flat Package (QFP) towards Ball Grid Array (BGA)/ChiP Size Package (CSP), which is a type of an area surface mounting package.
Among those types of packaging techniques, the Flip Chip-Ball Grid Array (FC-BGA) technique, which mounts a semiconductor chip on a printed wiring board having an interposer therebetween and a Cu electrode formed on the printed wiring board and a Cu electrode formed on the interposer are electrically connected by a solder ball, draws attention due to low cost compared to a mounting technique by wire bonding using a Au wire.
Surface treatment is performed on the Cu electrodes that are formed on the interposer and the printed wiring board such that each of the Cu electrodes is connected with high reliability. Examples of surface treatment may include Ni/Au plating, onto which gold (Au) plating is performed on a surface of an electrode on which nickel (Ni) plating has been performed. Further, in recent years, since mounting reliability by a solder ball is good, Ni/Pd/Au plating, in which Ni plating, palladium (Pd) plating, and Au plating are performed sequentially in order, has been widely spread.
In addition, as a solder mounting material, solder including no lead (Pb) has been used instead of conventional Sn—Pb based solder that is subject to RoHS restrictions. Particularly, a Sn—Ag—Cu based solder such as Sn-3Ag-0.5Cu has been widely used. However, the use of a Sn—Ag—Cu based solder entails the following disadvantages such as that when a component with a low heat resistance is mounted, thermal degradation of the component occurs because of an increase in reflow temperature or that when the solder is bonded onto a thin substrate, the substrate becomes bent because of heat. Particularly, for using FC-BGA, reflow should be performed multiple times based on, for example, the number of substrates to be laminated, and these disadvantages further limit a mountable component and the thickness of substrate. In addition, an intermetallic compound layer formed in a bonding interface between a plating film and a solder that are formed on an electrode grows thick because of the rise in reflow temperature, and this results in degradation of impact resistance. Thus, it has been required to lower a mounting temperature.
In order to lower a mounting temperature, solder having a low melting point is used. Such solders may include Sn-58 wt % Bi and Sn-57 wt % Bi-1 wt % Ag solders. Sn-58 wt % Bi and Sn-57 wt % Bi-1 wt % Ag solders have a low melting point of 139° C. and a mounting temperature of approximately 170° C. even at the peak. Thus, using these solders may lower the mounting temperature by approximately 60° C. in comparison with other Sn—Ag—Cu based lead-free solders and by approximately 30° C. in comparison with Sn-37 wt % Pb solder.
Sn-58 wt % Bi and Sn-57 wt % Bi-1 wt % Ag, however, are not widely used because of their hard and brittle properties that degrade impact resistance. However, there is an example of improving the ductility of solder by using Sn-57 wt % Bi-0.5 wt % Sb solder where Sb is added to Sn—Bi.
As described above, as the need for a solder with a low melting point increases, high reliability of solder mounting between the solder with a low melting point and a plating film is required. Particularly, for using FC-BGA, and the like, a reflow resistance that allows retaining high reliability of solder mounting even after many times of reflow is required.
Suggested is a joined body obtained by forming a thin film including at least one of Pd and Ni on Cu, bonding a solder thereon, and then removing the thin film (for example, Japanese Patent No. 4822526).
However, the solder includes 80 wt % to 100 wt % of Bi and has a bonding temperature of 270° C. or higher, and thus it is difficult to be used for components with low heat resistance.
The present invention takes into consideration of the above circumstances and provides a wiring substance capable of achieving highly reliable solder mounting even when a solder having a melting point of lower than 140° C. is used and provides a method for manufacturing the same.